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The Brainliest Answer!
2016-01-17T20:49:26+05:30
mass number includes mass of protons and neutrons only because mass of electron is negligible as compared to protons and neutrons therefore it is not included while calculating the mass number ...
 i hope it helps..........
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2016-01-17T21:01:18+05:30
Mass of a proton =mass o a neutron=1837 times the mass of the elctron
elctrons have negligible mass due to the speed of revolution around the nucleus
Δp=Δ(mv) 
change in momentum=change in (mass *velocity) of the electron
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